With the increasing cost of conventional energy sources, such as coal, oil and natural gas, attention has been directed to harnessing solar energy. Substantial gains have been made in the development of photovoltaic technology, but the cost per unit of usable energy continues to be excessive. Contributing factors of such high cost are found in the cost of production of photovoltaic devices--primarily, cost of material and man-hours for assembly--maintenance costs and reliability of extended performance.
Advances in the development of the photovoltaic cell itself have been forthcoming. Indeed, the production of amorphous solar cells, such as amorphous silicon cells, has considerably reduced material cost of photovoltaic assemblies. For a more detailed discussion regarding the development of amorphous solar cells, reference may be made to U.S. Pat. No. 4,409,605, to Ovshinsky et al.
Even with the reduction in material costs, the overall cost of photovoltaic devices remains high. This is primarily due to the man-hours involved in assembly of photovoltaic modules and arrays as well as maintenance and lack of proven extended reliability.
A photovoltaic module generally includes a plurality of photovoltaic cells, or similar power generating members, that are electrically interconnected in series and/or in parallel to produce the desired output voltage and current. When all the photovoltaic cells of the module are operating properly, the electrical output of the module properly represents the aggregation of the output of the individual cells. When, however, one or more of the cells experiences a reduction in output, either temporarily or permanently, the output of the module may be dramatically affected. For example, should one cell develop an open circuit, or otherwise become current limiting, power from other cells in series therewith will be restricted by the open circuit. Likewise, should one cell fail to generate power, for example it may be shadowed from the activating light, that cell may become reverse biased and thereby restrict the power output of all the cells electrically interconnected in series with it. Moreover, should the cell be only temporarily shadowed, such as by a leaf or other debris temporarily covering the cell, the electrical potential across the cell, as a result of reverse biasing, may cause the cell to be permanently damaged.
To obviate these problems, bypass or shunt diodes are employed in the module. These bypass diodes, generally, are connected across rows of parallel-connected photovoltaic cells, in parallel therewith. When all the cells are fully illuminated and producing energy, the bypass diodes are reverse biased and the current flow is through the cells. However, when current flow through any photovoltaic cell becomes limited, and thereby reverse biased, the parallel-connected bypass diode becomes forward biased, and current flow thus is conducted through the bypass diode and around the affected cell, thereby conditionally bypassing the affected cell and protecting the same from damage.
Ideally, a bypass diode should be associated with each photovoltaic cell or power generating member. However, such is not economically practical, as it is necessary to interconnect each diode with the corresponding cell and further to provide adequate heat dissipation for each diode while maintaining a relatively compact configuration of the photovoltaic module, in toto.
As such, it has generally been accepted in practice to employ one bypass diode for a plurality of interconnected cells, i.e., sub-modules. While this reduces production cost for the module, it detrimentally affects the performance thereof. Indeed, should one cell of the submodule experience a reduction of power output, the entire sub-module, with the remaining productive cells, may be conditionally electrically isolated, as a result of the bypass diode, from the power network of the module. Therefore, the output power of the module, as a whole, may be substantially reduced disproportionately as a result of the power loss of a single cell. And, as such, the module itself would be in need of substantial remedial repairs to restore the module to full performance.
In addition to the foregoing, the electrical interconnections between bypass diode and cell and between successive bypass diodes must be carefully considered. Indeed, these interconnections are subjected to a multitude of stresses resulting from mechanical loading and thermal cycling. As such, these electrical interconnections may experience fatigue failure during the life of the photovoltaic module, thereby necessitating remedial repairs.
Despite the substantial developments made regarding photovoltaic devices, no photovoltaic module or array provides an inexpensive and reliable assembly for conditionally electrically bypassing a power generating segment--either an individual photovoltaic cell or a plurality of cells--of the module. Namely, no module or array incorporates individual bypass diodes electrically associated with each photovoltaic cell in a manner that is both economical from a production aspect and reliable from an extended use aspect.